1. Field of the Invention
The present invention relates to a noise filter capable of removing noises generated due to a difference of a DC magnetic strength in an inner operation space of a magnetron and a collision with gases remaining in the inner operation space when electrons generated by a cathode rotate in the inner operation space, and in particular to a noise filter of a magnetron and a method for forming a noise filter which makes it possible to remove a noise generated at a high frequency band width higher than a few hundreds MHz by optimizing the length of a choke coil.
2. Description of the Background Art
Generally, in a magnetron, when a filament of a cathode terminal is heated by a power supplied thereto, and then thermal electrons are outputted, the thusly outputted thermal electrons circularly move by a magnetic field formed by a magnet installed in the inner space of the magnetron and an electrical field formed in a vertical direction with respect to the magnetic field, and radio frequency waves are outputted to the outside through a radio frequency output terminal.
When the magnetron is in an operation state, electrons which circularly move in an inner space of the magnetron colloid with the gases remaining in the operation space for thereby generating noises. At this time, the range of the noise is a few MHz to a few tens MHz. In order to remove the noises, a conventional magnetron noise filter, as shown in FIG. 1, is used.
As shown in FIG. 1, the conventional noise filter of the magnetron includes a shield box 10 fixed to a lower portion of the magnetron, a through type capacitor 20 installed by the shield box 10, a choke coil 40 for connecting a terminal of the through type capacitor 20 with a cathode terminal 30 of the magnetron, and a ferrite rod 50 installed in the choke coil 40 which will be explained in detail as follows.
The choke coil 40 is constructed that a first terminal unit 40-1 having a certain length is connected with the cathode terminal 30, a first bent portion 40-2 is extended and bent from the first terminal portion 40-1, a wound unit (choke coil 40-3) on which coils are wound many times for thereby having a certain diameter from the first bent portion 40-2, a second bent portion 40-4 is formed at an end of the wound portion 40-3, and a second terminal portion 40-5 is formed at an end of the second bent portion 40-4 and is connected with the terminal of the capacitor 20.
FIG. 2 illustrates an equivalent circuit of the magnetron noise filter of FIG. 1. As shown therein, there is provided an impedance ZL of the choke coil formed of an inductance component LL Of the choke coil 40, a resistance component RL which represents a power loss of the choke coil and a capacitance component CL. In addition, the impedance ZL is connected with a ground through the through type capacitor 20 for thereby forming a low band pass filter.
FIG. 3 illustrates an attenuating ratio characteristic curve of the equivalent circuit of FIG. 2. As shown therein, resonant bands are generated between about 300 MHz and 1 GHz as indicated by the doted line circle.
The noise filter having the above-described characteristics will be analyzed based on the equivalent circuit of the noise filter of the conventional magnetron, as shown in FIG. 2.
In the case of the relatively low frequency wave, the noise filter is capable of removing noises because the resonant point can be detected. However, when the frequency is increased up to a frequency higher than a few hundreds MHz, the noise filter can not effectively remove noises because the resonant point can not be detected and it is impossible to predict where a resonant point is generated according to the frequency increase.
FIG. 4 illustrates an Electro-Magnetic Interference (EMI) characteristic curve of a product in which the conventional magnetron is adapted. As shown therein, noises which exceed an EMI radiating standard reference is generated below 100 MHz, around 500 MHz, and 700xcx9c800 MHz.
In order to overcome the problems of the above-described conventional noise filter, a coil is additionally provided to the choke coil of the magnetron for thereby forming a filter (not shown) having a two-tier coil structure in accordance with another conventional embodiment. Namely, in order to increase the noise attenuating capability of the noise filter in accordance with the first embodiment, the number of turns of the choke coil is increased for thereby increasing the impedance. In this case, the size of the choke coil is increased. A margin with respect to a safe distance is decreased due to a high Voltage. In this case, it is impossible to continuously increase the size of the choke coil for increasing the noise removing capability of the noise filter. Further, in order to increase the impedance, when the size of the choke coil is increased by increasing the number of turns of the choke coils, the temperature of the choke coil is increased for thereby causing temperature loss. Therefore, in order to satisfy the safe distance and a temperature increase condition and to remove for a radiation noise, an experiment must be performed for filtering noise with respect to all the magnetrons, respectively. Even when the above-described conditions are obtained based on the experiment, it is impossible to fabricate the magnetron noise filter which satisfies the above-described conditions. Therefore, a coil is additionally provided to the choke coil of the magnetron for thereby forming a 2-tier noise filter structure.
However, in the first embodiment of the conventional art, the noise filter formed of the choke coil and the through type capacitor has a problem that a noise generated below 100 MHz, near 500 MHz and at the band width of 700xcx9c800 MHz is not removed. In addition, in the two-tier noise filter, having an additional choke coil, in accordance with the second embodiment of the conventional art, it is impossible to perfectly remove the noise, and the price of the same is high.
Accordingly, it is an object of the present invention to provide a noise filter of a magnetron which is capable of removing noises generated at a high frequency bandwidth.
It is another object of the present invention to provided a method for forming a choke coil in a noise filter of a magnetron which is capable of removing noises generated at a high frequency bandwidth.
To achieve the above objects, there is provided a magnetron noise filter which includes a shield box fixed to one side of the magnetron, a through type capacitor installed at one side of the shield box and a combined choke coil connected to a cathode terminal of the magnetron and a terminal of the capacitor, wherein the combined choke coil comprising a tightly wound portion around a bar having a certain diameter and a loosely wound portion connected with the tightly wound portion.
To achieve the above objects, there is provided a method for forming a noise filter of a magnetron in which a filter including a cathode terminal of a magnetron, a through type capacitor and a choke coil is connected with the cathode terminal. The method includes a step of obtaining a certain resonant point at a high frequency band width, a step of setting a length of a physical copper line of a choke coil for enhancing an attenuating ratio of a resonant frequency with respect to the resonant point, a step of forming a tightly wound portion by winding a copper line having a set length onto a certain ferrite, a step of obtaining a resonant point with respect to the oscillation frequency reflected from the interior of the magnetron, and a step of setting the length of a physical copper line for thereby obtaining the resonant point and forming a loosely wound portion at a portion having a certain distance from the tightly wound portion.